Please help me not make a dumb mistake

Yes it is the same!
A "good ground" is a ground circuit that has a resistance of zero OHMS.

Is there a reason that you don't want to use the low level inputs? I've always found that using the low level inputs sounds better to me. The REL "experts" disagree however. You can use the high level inputs, but that won't allow you to limit the low frequencies to speakers.

Thank you, Yogiboy. I appreciate the help.

Fiesta75-- I don't have the option of using the low level subwoofer input in this system as it is currently configured, which is:

Mac mini --> Benchmark Dac3 (dac/preamp) --> Prima Luna Dialogue 4 --> Dali S180.

I have used the low level inputs on other subwoofers in different systems with good results. This will be my first experience using a subwoofer's high level input. Hopefully it works out.

I think you will find that speaker level inputs are better and it’s easier to integrate the sub with the main speakers.

Oz

Your speaker level inputs share a ground common between left and right. Amplifiers that use differentially balanced circuits have separate grounds for left and right. I'm pretty sure the PL amplifier is a single-ended amp, so you should be fine, but it might be wise to check with your dealer or the manufacturer just to be sure.

My sub is hooked up to my Prima Luna speaker outputs...no problem. 

It will work fine. If you are super worried then call PL. Or even better simply remove the bottom cover and look inside. There you will see it is wired the way cleeds said, everything connects to the same ground. Tube amps are not crammed with lots of parts it will be easy to follow the wires, and see the - for both channels connects to the same ground. 

Ozzy62-- As you said, I have heard many people report that speaker level inputs can result in a more seamless integration. I hope that turns out to be true in this system. In the past, I have spent many hours trying to get subs to integrate using the low level inputs. I have sometimes achieved good results with the help of digital EQ and experiments in room placements. In this system, I will have no EQ capability and only a little latitude with room placement.

Cleeds-- Thank you for the good advice. I have reached out to the manufacturer and am waiting to hear back. I'm feeling more confident that it won't be a problem given russ69's experience. 

Russ69-- Thanks for your comment. It sounds like it will work fine.

I appreciate the help folks.